nutrient testing for soils, potting mixes and hydroponics.
Many people want to know if there are any reliable and easy ways to test nutrients in their own soils.
Yes, for accuracy and convenience its worth investing in one of the ready made kits. Examples can be seen in the range of Lamotte kits for soils and hydroponics. See them at Lamotte soils and hydroponics kits. But there are some tests that can be done with minimal preparation that can still produce useful results.
There are two basic questions about what nutrients are in a soil. How much is potentially available and how much is in the soil solution in contact with the roots? Potentially available nutrients are held in the soil organic matter and in loose associations with other soil minerals. To estimate them the soil is mixed with various solutions called extractants designed to capture the different nutrients. This process and the calculations that follow normally require at least a modest lab setup so that soil can be weighed and extractions can be done and a little time.
Luckily there are some simple ready-to-go solutions. Both Hanna Instruments and Lamotte make NPK and pH kits with extractants. The Hanna kit is the HI3896 Agricultural NPK / pH test kits. See details at Agric NPK / pH test kit. A good kit for both the serious home gardener and professional is the Lamotte EL garden kit. This kit performs NPK and pH tests. See is at Model EL Garden kit. Both kits contain full instructions and notes to help with interpreting results.
A further range of kits is available to test for both major and minor nutrients. For example:
Lamotte SCL-12 Electronic soil test lab. If you want an excellent completely self contained colorimeter based soil test kit then we can supply the Lamotte SCL-12 or SCL-15 electronic soil test labs. See it at Lamotte electronic soil lab.
Readily available nutrients
Nutrients readily available to plants are already in the soil solution but if a soil is dry then the nutrients will be more concentrated than if the soil is wet. To get a figure that will be useful for comparisons between soils and over time, a compromise, if the soil is not already moist is to add distilled water until the soil is just saturated, then test the solution. Test strips are a convenient, easy and cost effective way to estimate some of the main nutrients in soils, soilless mixes and in hydroponics. They test the readily available nutrients. For more reliable results in agricultural soils some distilled water should be added to the soil for example in the ratio 5 : 1, distilled water : soil. After the nutrient is tested that value is then converted to the actual concentration in the soil solution by a relatively simple calculation. For accuracy you will need to measure or estimate the % water content of the soil. Calculations and methods are available for nitrates in soil and for converting conductivity measurements (a measure of nutrient supply). Until the methods are available here please email us for the details. See Easy soil tests you can do.
Here are some tests that will get you well on the way towards understanding the fertility of your soil.
On this website there are some useful notes on understanding soil test results in MS Word format. They explain how soil tests are carried out, how to interpret test results and how testing relates to soil fertility management.
A quick primer: There are 3 types of acidity in soils. 1. H+ in the soil solution (measured with a pH probe or test strips).
2. H+ and aluminium hydroxides (the larger proportion) loosely held on the soil mineral and organic matter. The hydroxides react with water in the soil to form H+ in the soil solution. This reaction is in dynamic equilibrium so can move back to aluminium hydroxides or forwards to H+. If a strong salt like KCl is mixed with the soil, theory suggests that the hydroxides are displaced into the soil solution and hydrolize to form H+. In the lab this can be neutralised with an alkali and is a measure of exchangeable acidity by salt extraction. Whilst this figure doesn't indicate how much lime to apply to raise pH it is useful as a way to monitor soil management activities that aim to raise pH. Hanna Instruments have a kit that measures Total Exchangeable acidity. See it at TEA kit.
3. Acidity (H+) that is tightly bound to the soil matrix. This can be displaced and replaced in soil by adding nutrient cations like calcium, magnesium (often as lime) and potassium. For a more complete explanation of this issue and its relationship to liming see Soil test interpretations.
Calcium and magnesium Measuring calcium and magnesium in soils is challenging. Hanna Instruments make a photometer based method that will measure Ca and Mg separately. But this method needs additional calculations to convert from concentrations in an extraction solution, to concentration in the soil weight for weight. See it at calcium and magnesium kit. Hanna Instruments also makes a chemical test kit HI38035 that can be used to measure Ca and Mg separately. We use this kit in the lab and it works well. We have also adapted it to Ca and Mg in soils through a double acid extraction step plus calculations. Contact Apps Labs for methods and calculations. See Easy soil tests you can do.
Sodium and gypsum requirement.
Is there sodium in your irrigation water? Continued use of irrigation water with an imbalance between sodium and the other more desirabe nutrients can lead to a soil with depleted nutrients and poor structure. Sometimes the remedy for saline soils is to apply gypsum. This is the basis for the Gypsum requirement /exchangeable sodium test from Hanna Instruments. The kit uses calcium sulphate to displace the sodium followed by a simple titration to find out how much of the calcium was used up. Very neat! See details at Hanna Instruments Exchangeable sodium / gypsum requirement kit.
This is a very useful measurement in soils because, amongst other things, pH and nutrient availability are connected. The easiest way to measure pH in soil is to use test strips. The pH-Fix strips at pH 4.5 to 10 will cover a suitable range. They are best used in moist soil. See them at pH test strips.
Readily available acidity, measured as pH can be tested with an electronic pH meter or tester. A pH tester that can measure pH in a slurry or in moist soil will give the most reliable results.
The conductivity of a soil is a useful measure of overall nutrients because it approximates the salts, good and bad in the soil solution. It may be useful at first to send off a sample to the lab to get a figure for chloride and sodium. These are the bad salts but once they are known then the rest of the conductivity measurement usually indicates the good nutrients and salts. Subsequent measurements can be used for monitoring changes in nutrients assuming there is no significant change in undersirable salts. There are several very good electronic conductivity testers available that can be used in a soil slurry. See a selection at Conductivity testers. The most convenient way to measure conductivity is in a soil : distilled water mix. A 1 : 5 mix is convenient. The conductivity measurement is then converted to actual conductivity in the soil solution using a relatively simple calculation that takes into account soil moisture content. Contact us for details on how to do this. See Easy soil tests you can do.
In hydroponics solutions nitrate should ideally be around 620 ppm with a minimum of 400 ppm. The MN Quantofix nitrate strips should show around 500 ppm in hydroponics solutions. Remember that to measure higher values a fertilizer solution can be diluted using distilled water (from the supermarket). Try a 1 : 1 dilution. In soilless mixes, the water extractable nitrate should be around 150 ppm. 15 to 25 ppm is a low nitrate value. For adequate nitrate in soilless mixes the MN test strips should show at least 100 ppm. 250 ppm is possibly a high value for most soilless mixes. If the MN strips show 10 ppm or less then nitrate levels are low in that mix. For agricultural soils test the nitrate level in a soil slurry (often the test is made clearer using a 5 : 1 dilution then an adjustment calculation). A value around 150 ppm is good for nitrates in soils for good plant growth.
Read about Macherey Nagel Quantofix nitrate test strips at Nitrate test strips.
Update: If you are testing nitrate in soils keep this in mind: The Macherey Nagel test strips are best for hydroponics solutions, fertilized soilless media, composts and worm castings mixes but for normal agricultural soils the Hach Nitrate test strips have a range best suited to the expected relatively low nitrate levels. Start by testing in a 1 : 1 soil : distilled water slurry. If you exceed the range on the strips then try a greater dilution eg 5 : 1. The actual concentration of nitrate in the soil solution can then be calculated based on the dilution factor in the soil : water mix and the percentage water in the soil (measured or estimated). See Easy soil tests you can do.
Use MN Quantofix phosphate test strips in hydroponics solutions. Dilute the solution with an equal amount of distilled water. If the strips read 100 ppm then this is actually 200 ppm in the solution which should be a high enough level of phosphate. An actual value of around 100 ppm phosphate is around minimum phosphate for hydroponics solutions. For soilless mixes a low phosphate value is around 5 ppm with medium levels around 12 ppm. Good phosphate levels are around 25 ppm. The MN Quantofix test strips will show these levels well because the gradation of the strips is 0 - 3 - 10 - 25 - 50 - 100 ppm.
Using this kit is a simple 2 step process using the reagents and tubes supplied and takes about 2 minutes.
Read about Macherey Nagel Quantofix phosphate test strips at Phosphate test strips.
MN Quantofix potassium test strips test in the range 0 - 1500 ppm with gradation 0 - 200 - 400 - 700 - 1000 - 1000 - 1500 ppm. Hydroponics solutions should contain from approximately 200 to 400 ppm with 300 ppm being optimal. The Quantofix potassium strips can show the minimum to maximum range well. In soilless mixes, if the strips show 200 ppm then this is around desirable. If they show 400 ppm then there is more than adequate potassium.
Using the Quantofix potassium test strips is a simple 2 step process. All reagents and vials are sullpied in the kit. Read about Macherey Nagel Quantofix potassium test strips at Potassium test strips.
Adequate sulphate levels in hydroponics solutions are around 200 ppm. In soilless mixes suphate levels around 400 ppm are considered high. The Quantofix sulphate test strips show sulphate levels around 200 ppm and less and sulphate levels around 400 ppm and higher at the lower end of their testing range.
Read about Macherey Nagel Quantofix sulphate test strips at Sulphate test strips.
This article is for general information only and information and advice contained in it is designed to encourage people to experiment with fertility testing kits as part of their production program. It is advisable that managers should refer to appropriate up to date information and guidelines or obtain professional advice regarding their specific situation for example type of crop.
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